This invention relates to a corrosion resistant lid used to close a semiconductor package and, in particular, to a lid having a novel protective coating composition.
In many semiconductor applications, the semiconductor device is required to be hermetically sealed within a "package" i.e., a ceramic case or housing that protects the device from the surrounding ambient and thus enhances its reliability. As explained in further detail in U.S.Pat. Nos. 3,340,602; 3,538,597; 3,874,549 and 3,946,190, the package typically includes a container having a cavity in which the device is securely mounted. Electrical leads are passed out of the container and are connected to appropriate circuitry. The package is sealed closed using a lid which is placed in registry over the cavity. The lid is hermetically sealed in place using a eutectic solder frame formed of an alloy made up e.g. of 80% gold and 20% tin.
The lid that is used throughout the industry is typically made from a Kovar stamping. Kovar is a well known alloy containing various amounts of cobalt, nickel and iron. Lids have heretofore been provided with a top coating of gold that is placed over an inner or barrier layer of nickel. This coating provides an excellent bonding surface for the solder frame and also provides a corrosion resistant shield for the Kovar substrate which, because it contains iron, is highly susceptible to rust damage. The nickel interface usually consists of between 50 and 150 microinches of low stress nickel while the top coat consists of about 50 microinches of a noble metal, i.e., pure gold. Although this dual combination exhibits good solder ability, the lid nevertheless will often rust when exposed to a corrosive atmosphere for an extended period of time. Corrosion in amounts of between 2-4% of the total surface area of the lid will generally occur within 24 to 96 hours when the coated lid is exposed to a salt-containing atmosphere.
The accepted industry standard which governs the amount of corrosion allowable for high reliability packages are set out in the military specification Mil. Std. 883 C. This specification has been recently revised so that all lids now must remain corrosion free (zero corrosion) after being exposed to a salt-containing atmosphere for at least 24 hours. Dual coated lids found in the prior art, i.e., those having a single lower coating of nickel and a top coating of gold, continually fail this corrosion test.
All electroplated metal coatings exhibit porosity to some extent and thus permit rust producing agents to pass therethrough to the base metal. Methods have been tried with varying degrees of success to reduce the porosity of protective coatings and to increase the resistance of these lids to corrosion. Porosity is usually inversely proportional to the thickness of an electroplated metal and the pores that initially form in the coating material close gradually as more metal is deposited. As the coating thickens, the pores eventually close. Approximately 2000 microinches of nickel and about 100 microinches of gold are required, however, to completely close the pores on a dual coated lid. The consumption of this amount of metal is not only expensive, but also requires an extraordinary amount of time to complete the plating process.
Pulse plating has also been tried with some limited success in an effort to close the pores in the coating materials. In this process, the current applied to the electroplating tank is pulsed on and off periodically by a square wave generator. The pulsing provides for increased ion mobility in the bath which, in turn, results in a smaller more densely packed crystal structure in the electroplated metals. This denser deposit is believed to fill the pores more rapidly and thus provide greater protection for a given coating thickness. Although the amount of corrosion may be reduced by this technique, pulse coating alone cannot provide economically feasible products capable of meeting the new standard within the industry.
More recent approaches to solving the porosity problem are set forth in U.S. Pat. No. 4,601,958, granted July 22, 1986 and U.S. Pat. No. 4,620,661, granted Nov. 4, 1986. Both of these patents disclose closures or lids for semiconductor packages that are coated with plural alternate electroplated layers of nickel and gold. Thus, the substrate of Kovar or the like is coated with a first layer of nickel, then a first layer of gold, then a second layer of nickel, followed by a second layer of gold. The resultant plated lids have been shown to have substantially higher resistance to salt atmosphere corrosion than lids having but a single layer of nickel and a single layer of gold.
Despite the improved results obtained by the multi-layer approach, some problems remain. One of these is the difficulty in recovering gold in the first or intermediate gold layer of rejected, sub-standard plated lids because no effective stripping agent for nickel is presently available. Another problem is that the second or outer nickel electroplate layer can be stressed causing blistering and separation from the gold layers.